Salad lore

The reason that the French and Italians eat salad at the end of their evening meal is because the ancient Romans and Egyptians took advantage of the natural opiate-like substance lactucarium,* present in all types of lettuce, to induce sleep. A good lettuce variety reference page is:
University of Wisconsin, Department of Hortoculture

* lactucarium, English lettuce and French laitue all come from the Latin word for milk (lac, lactis), also related to Greek galax (whence our word for galaxy or the Milky Way). This is because, what grew in different varieties as mostly weeds all around the Mediterranean and that people began to eat, contained a milky liquid in the lower parts and stems of the plant, the most obvious example an American knows being the dandelion (dents de lion or lion’s teeth by reason of the shape of the leaf), whose leaves the French commonly eat in salads today.